This invention generally relates to a circuit for gating DC power, and is specifically concerned with a low loss pulser circuit for a solid state microwave power amplifier.
Solid state amplifiers are replacing traveling wave tube type amplifiers in modern radar transmitters. In the gigahertz range, these amplifiers are typically GaAs FETs. A typical X-band power amplifier output stage may draw 3 amps at 10 volts. For a variety of reasons, it is desirable to apply the 10 volt power signal to the drain of the microwave amplifier only during a transmit pulse. Transmit pulses may be repeated at a rate of a few hundred kilohertz and require rise and fall times of 20 nanoseconds or less.
The 10 V DC power signal is applied to the drain of the microwave amplifier by means of a drain pulser circuit. This circuit acts as a switch connecting the amplifier drain to a DC power source during each transmit pulse and quickly pulling the amplifier drain voltage to ground between pulses. This function is easily accomplished using a circuit having a power MOSFET as the switch. Circuits using a power MOSFET as a switch are well known in the art. The circuit which is the subject of this invention employs a power MOSFET, but includes a unique bias and gate drive configuration with advantages which will become apparent. The circuit of the invention has great utility as a drain pulser for a solid state transmitter, but may find application in other situations requiring fast switching of a DC power voltage.
In many prior art circuits for gating the power to the drain of a solid state amplifier, such as the circuit in FIG. 1, a P-channel power MOSFET (100 in FIG. 1) is used as the drain switch. Vout in FIG. 1 is connected to the drain of a microwave amplifier. Use of a P-channel power MOSFET is convenient because it can be turned on by pulling its gate voltage to Ground. No additional bias voltages are required. However, the use of an N-channel power MOSFET as the drain switch in this application would be more desirable since the lower on-state resistance of the N-channel MOSFET would result in lower power losses and higher efficiency. Additionally, N-channel MOSFETs can be made smaller for equivalent on-state resistance, and so have faster switching capabilities. Unfortunately, N-channel MOSFETs have been considered impractical in this application since the gate bias voltage required is more positive than the input power voltage. For example, if the input power voltage were 10 V, a bias voltage of 15 or 20 V would be required at the gate terminal of a N-channel power MOSFET drain switch. The additional expense and effort associated with providing this extra power supply voltage and connection is highly undesirable.
Some prior art circuits have attempted to use an N-channel power MOSFET in a common drain connection (as is required here) for switching through the use of a control circuit which generates the needed gate bias potential through a charge pump scheme. One such prior art circuit is shown on page 77 of "High Frequency Switching Power Supplies" by George Chryssis, McGraw Hill, 1984, and reproduced here in FIG. 2. The N-channel Power MOSFET 200, which would be the drain switch, acts as the output switch, while transistor 202 pulls down the output when SVin goes high. Transistor 204 acts to pull down the gate of the drain switch transistor 200 when SVin goes high. When SVin goes low, the gate of 200 is pulled up by resistor 208 and the drain switch starts to turn on. The capacitor in this circuit acts as a charge pump providing positive bias and turning 200 fully on.
However, this circuit has several problems that prevent proper function of the circuit in a high-pulse rate radar application. First, during the off-state (when SVin is low), the input voltage Vin is applied across resistor 208. This wastes power and forces the use of a large value resistor 208, considerably slowing the switching speed. Second, turn-on of this circuit occurs in a two-step process since the charge pump gate drive is bootstrapped from the output. The two-step turn-on increases the response time of the circuit. Finally, shoot-through currents can occur between drain switch transistor 200 and transistor 202, wasting significant power during high frequency operation. Thus, the circuit of FIG. 2 will not operate at the high pulse rates required for modern radar applications.